The invention concerns a polyester fibre composition and a process for its manufacture which utilises a novel organotitanium or organozirconium catalyst
Antimony (Sb), tetraisopropyl titanate, and triethanolamine titanate are known catalysts for esterification processes. Also, organotitanium compounds and, in particular, titanium alkoxides or orthoesters are known as catalysts for esterification processes. Many organotitanium compounds which are effective catalysts in the manufacture of polyesters such as polyethylene terephthalate are known to produce unacceptable yellowing in the final polymer. U.S. Pat. No. 5,866,710 describes an esterification process using a catalyst system which comprises the reaction product of an orthoester or condensed orthoester of titanium or zirconium, an alcohol containing at least two hydroxyl groups, a 2-hydroxy carboxylic acid and a base. The polyesters produced by such a process show a reduced amount of haze and yellowing in comparison to a known titanium isopropoxide catalyst U.S. Pat. No. 5,866,710 teaches that the resulting polyesters are useful in films and bottles; the reference does not teach or suggest using the resulting polyesters in fiber or yarn.
When polyester articles are formed from molten polyester, when processing polyesters into textile fibres or bottles for example, the polymer is melted and may be held in the molten state for a period of time before being shaped by e.g. spinning or injection moulding. Two key rheology measurements: shear viscosity or complex viscosity as a function of shear rate or frequency and extensional viscosity as a function of shear stress are used to characterize polyesters. Zero-shear viscosity is typically taken as an indication of polymer molecular weight while the transient extensional viscosity is an indicator of the polymer's extensional response to stretching.
We have now found a catalyst composition for producing polyesters in particular which exhibit unexpectedly improved melt rheological properties compared with polyester of the same intrinsic viscosity made using known catalyst systems, and which are therefore particularly suitable for making polyesters for such applications.